9. september 2010 thomas bergauer (hephy vienna) operation of the cms tracker at the large hadron...

9. September 2010

Thomas Bergauer (HEPHY Vienna)

Operation of the CMS Tracker at the Large Hadron Collider

ÖPG/FAKT Annual Meeting Salzburg


CMS: Compact Muon Solenoid

Weight: 12.500 tDiameter: 15 Length: 21.5 m

Weight: 12.500 tDiameter: 15 Length: 21.5 m

SupraconductingMagnet (4 T)

HadronicCalorimeter

ElectromagneticCalorimeter

SiTracker (Pixel and Strips) Muon System

Magnet Return Yoke

Very ForwardCalorimeter

2Thomas Bergauer (HEPHY Vienna)9. September 2010


CMS Silicon Strip Tracker

• Largest silicon tracker built• Active area of 198 m2

– 5.4 m long, 2.4 m diameter

• Components:– Pixel detector

(not covered in this talk)

– TIB (Inner barrel): 4 layers

– TID: 3 Inner Disks

– TOB: (Outer Barrel): 6 layers

– TEC (Endcaps): 9 disks on each side

• Key features:– 9.6 Million readout channels

– Analog readout


Barrel(BPIX)

Endcap(FPIX)

L ~ 90 cmrmin = 4.4 cmrmax = 10.2 cm


CMS Tracker in pictures



CMS Tracker Installation December 2007



• Angular coverage down to 9 degree to the beam-pipe (|η|<2.5)

• 4 layers and 3 rings contain stereo modules for 2D hit reconstruction

Basic Building Block:

Detector module• 15 148 pieces in total• 15 different geometries• Modules consist of– Carbon fiber/graphite frame– Front-end hybrid with APV25

readout chips– One or two p-on-n silicon

sensor(s), 320/500 micron thick

Thomas Bergauer (HEPHY Vienna)9. September 2010 6


Readout Chain



Commissioning Procedures

• Analog readout– Digitization is done only in off-

detector electronics (FEDs)

• Thus, detector needs to– Time-align internally (different

cable lengths)– Tune laser gain (analog opto-

hybrids)– Optimize chip parameters

(baseline,…)– Determine noise and

pedestals (zero-suppressed data)

• Benefit of analog readout– Higher position resolution– Makes debugging easy



Operational fraction of SST

• 98.1% of channels in operation

• TIB/TID: 96.3 %– One ring lost (short,

appeared with B field), ~1%

– HV missing and HV shorts, ~2.5%

• TOB: 98.8 %– One ring lost (short,

comes/goes with B field)

• TEC: 99.0 %– One HV PG missing (short)– One LV PG missing (short)


TID+ TEC+

TID- TEC-

TIB TOB


Signal-to-Noise Ratios


TIB TID TOBTEC thin

TEC thick

19.4 18.5 22.5 19.1 23.4

• Charge clusters of

associated tracks– Divided by noise determined

during calibration (pedestal) run

– Non-perpendicular tracks

normalized by trigonometry

• Landau convoluted

with Gaussian – MP value taken for summary


Tracker Alignment• 15148+1440 sensors

– 6 degree of freedom each

• O(10m) accuracy

• Minimization hit/track residuals 2

• Two approaches:– Millipede (II): Global minimization– “Hits and Impact Points” (HIP): local

minimization of sensor position, iterative, detailed track model

– Kalman Filter-based fit method working on “correlated” elements, iterative

– Applied sequentially • from large substructures to sensor level

• Distributions of Mean Residual (DMR): median of the residual distributions in each sensor

• 2010 cosmics and collision events used for present alignment:

– 1.5M cosmic tracks (p>4 GeV)– 1.7M collision tracks (p>3 GeV)

• with constraint to primary vertex

p>3 GeV/cp

T>0.65GeV/c

Only modules with >200 hits



Summary

• CMS Tracker (together with whole CMS experiment) performs excellently in both cosmics and pp collision runs

– 98,1 % channels in operation

• Tracker uses analog readout from detector to off-detector electronics

– Makes different calibration runs necessary

• Signal-to-noise ratio meets expectations

• Alignment algorithms reveal accuracy of 10μm

• Tracker contributes to the high quality physics data CMS delivers



THE ENDThank you for your attention



APV25 Peak vs. Deconvolution mode


Deconvolution mode– output charge for each strip represents a

weighted sum of three consecutive pipeline cells

– designed to avoid signal pile-up in high luminosity operations

– necessary whenever bunch separation is less than a few hundred nanoseconds


Collected Events

• pp Collisions– Dec 2009 (900GeV+2.36 TeV):

~300k MinBias Events– 2010 (7 TeV): ~3000 nb-1

• Cosmics muons– 2008: 3M tracks in tracker– 2009: 4M tracks– 2010: 2.2M tracks

• ~ 4% in pixel detector volume

• alignment, calibration, noise, resolution



Track Reconstruction Efficency

Tracks reconstructed in three steps:• seeding: hit triplets (mainly pixel hits)

or pairs + beam spot used as track candidate

• Pattern recognition: track candidate propagation (Kalman filter), addition of new compatible measurements, track candidate cleaning

• Final Track Fit: track parameter estimator

• Track Selection: fakes rejected with quality cuts

Iterated several times:• hits associated to reconstructed tracks

are removed• different seeding algorithms• different quality cuts



Impact parameter


transverse longitudinal

9. september 2010 thomas bergauer (hephy vienna) operation of the cms tracker at the large hadron...

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thomas bergauer hephy

cms experiment

high quality physics

pixel detector volumealignment

pixel hits

readout channelsanalog

talktib inner barrel

noise ratio